This invention relates to a fluidized bed combustion system and a process of operating same and, more particularly, to such a system and process in which dampers control upper furnace solids loading while maintaining full load stoichiometry at lower loads.
Fluidized bed combustion systems are well known and include a furnace section in which air is passed through a bed of particulate material, including a fossil fuel, such as coal, and an adsorbent for the oxides of sulfur generated as a result of combustion of the coal, to fluidize the bed and to promote the combustion of the fuel at a relatively low temperature. These types of combustion systems are often used in steam generators in which water is passed in a heat exchange relationship to the fluidized bed to generate steam and permit high combustion efficiency and fuel flexibility, high sulfur adsorption and low nitrogen oxide emissions.
The most typical fluidized bed utilized in the furnace section of these type systems is commonly referred to as a "bubbling" fluidized bed in which the bed of particulate material has a relatively high density and a well-defined, or discrete, upper surface. Other types of systems utilize a "circulating" fluidized bed in which the fluidized bed density is below that of a typical bubbling fluidized bed, the fluidizing air velocity is equal to or greater than that of a bubbling bed, and the flue gases passing through the bed entrain a substantial amount of the fine particulate solids to the extent that they are substantially saturated therewith.
Circulating fluidized beds are characterized by relatively high internal and external solids recycling which makes them insensitive to fuel heat release patterns, thus minimizing temperature variations and, therefore, stabilizing the sulfur emissions at a low level. The high external solids recycling is achieved by disposing a cyclone separator at the furnace section outlet to receive the flue gases and the solids entrained thereby from the fluidized bed. The solids are separated from the flue gases in the separator and the flue gases are passed to a heat recovery area while the solids are recycled back to the furnace through a seal pot or seal valve. All of the fuel is combusted and the heat of combustion is absorbed by water/steam-cooled tube surfaces forming the interior boundary of the furnace section and the heat recovery area. The recycling improves the efficiency of the separator, and the resulting increase in the efficient use of sulfur adsorbent and fuel residence times reduces the adsorbent and fuel consumption.
In these type of arrangements, the amount of primary air supplied to the fluidized bed must be limited to that below the ideal amount for complete combustion in order to reduce nitrous oxide (NOX) emissions. Thus, overfire or secondary air is injected above the fluidized bed in sufficient quantities to maintain a ratio of primary air to secondary air to insure complete combustion.
However, problems arise in maintaining this requisite ratio of primary air to secondary air during low load conditions. More particularly, as load is reduced the solids circulation, or loading is also reduced which reduces the residence time of the solids and the capture of sulfur oxides (SO.sub.2). Our solution to this is to increase the amount of primary air. However, this destroys the requisite ratio of primary air to secondary air, resulting in increased NOX emissions.
Also in these types of fluidized beds, particulate fuel of a size extending over a relative wide range is utilized. For example, a typical bed will contain relatively coarse particles of 350-850 microns in diameter which tend to form a dense bed in the lower furnace, and relatively fine particles of 75-225 microns in diameter which are entrained by the flue gases and recycled. This tends to reduce coarse particle entrainment and cause instability in the dense bed of coarse materials resulting in sluging or choking of the bed material and pressure fluctuations in the lower furnace.